IF-THEN-ELSE statement in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the input is ON and set the output accordingly.

PLC Programming

IF input [1] TRUE THEN
    output := TRUE;
ELSE
    output := FALSE;
END_IF;

Drag options to blanks, or click blank then click option'

A==

B=

C:=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=' for comparison.

Using ':=' which is for assignment, not comparison.

In PLC programming, the IF statement uses '=' to compare values, not '==' or ':='.

2fill in blank

medium

Complete the code to check if temperature is above 100 and activate alarm.

PLC Programming

IF temperature [1] 100 THEN
    alarm := TRUE;
ELSE
    alarm := FALSE;
END_IF;

Drag options to blanks, or click blank then click option'

A>

B>=

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which checks if temperature is less than 100.

Using '>=' which includes 100, but the task wants strictly above.

We want to activate the alarm when temperature is greater than 100, so use '>'.

3fill in blank

hard

Fix the error in the IF statement to correctly check if pressure equals 50.

PLC Programming

IF pressure [1] 50 THEN
    valve := OPEN;
ELSE
    valve := CLOSED;
END_IF;

Drag options to blanks, or click blank then click option'

A:=

B!=

C=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':=' which causes syntax errors in IF conditions.

Using '==' which is not standard in PLC languages.

In PLC languages, '=' is used for comparison, not ':=' or '=='.

4fill in blank

hard

Fill both blanks to check if speed is less than 60 and set status accordingly.

PLC Programming

IF speed [1] 60 THEN
    status := [2];
ELSE
    status := STOPPED;
END_IF;

Drag options to blanks, or click blank then click option'

A<

BRUNNING

C>

DIDLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' for the speed check.

Setting status to IDLE instead of RUNNING when speed is less than 60.

We check if speed is less than 60 using '<'. If true, status is set to RUNNING.

5fill in blank

hard

Fill all three blanks to create a nested IF that checks temperature and pressure.

PLC Programming

IF temperature [1] 80 THEN
    IF pressure [2] 30 THEN
        alarm := [3];
    ELSE
        alarm := FALSE;
    END_IF;
ELSE
    alarm := FALSE;
END_IF;

Drag options to blanks, or click blank then click option'

A>

B<

CTRUE

DFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' for temperature check instead of '>'.

Setting alarm to FALSE inside nested IF when conditions are met.

The code checks if temperature is greater than 80 and pressure less than 30, then sets alarm to TRUE.

Practice

1. What does an IF-THEN-ELSE statement do in PLC programming?

easy

A. It repeats a set of instructions multiple times.

B. It sends data to an external device.

C. It stores data in a table format.

D. It chooses between two actions based on a condition.

Solution

1. Step 1: Understand the purpose of IF-THEN-ELSE

   The IF-THEN-ELSE statement checks a condition and decides which action to take based on that condition.

2. Step 2: Compare with other options

   Repeating instructions is done by loops, storing data is done by variables or tables, and sending data is communication, not decision making.

3. Final Answer:

   It chooses between two actions based on a condition. -> Option D

4. Quick Check:

   IF-THEN-ELSE = Choose action based on condition [OK]

Hint: IF-THEN-ELSE picks one of two paths [OK]

Common Mistakes:

• Confusing IF-THEN-ELSE with loops
• Thinking it stores data
• Assuming it sends data externally

2. Which of the following is the correct syntax to start an IF statement in PLC structured text?

easy

A. IF condition THEN

B. IF condition DO

C. WHEN condition THEN

D. IF condition THEN DO

Solution

1. Step 1: Recall PLC IF statement syntax

   The correct way to start an IF statement is with 'IF condition THEN'.

2. Step 2: Check other options for errors

   'DO' is not used after IF in PLC structured text, 'WHEN' is for CASE statements, and 'THEN DO' is invalid syntax.

3. Final Answer:

   IF condition THEN -> Option A

4. Quick Check:

   IF starts with 'IF condition THEN' [OK]

Hint: IF always pairs with THEN, no DO needed [OK]

Common Mistakes:

• Adding DO after THEN
• Using WHEN instead of IF
• Mixing CASE syntax with IF

3. What will be the output of this PLC code snippet?

IF SensorInput = TRUE THEN
  Motor := ON;
ELSE
  Motor := OFF;
END_IF;

Assuming SensorInput is FALSE initially.

medium

A. Motor will keep previous state

B. Motor will be ON

C. Motor will be OFF

D. Syntax error occurs

Solution

1. Step 1: Evaluate the condition with SensorInput = FALSE

   The condition 'SensorInput = TRUE' is false, so the ELSE branch runs.

2. Step 2: Determine the action in ELSE branch

   In ELSE, Motor is set to OFF.

3. Final Answer:

   Motor will be OFF -> Option C

4. Quick Check:

   Condition false -> ELSE -> Motor OFF [OK]

Hint: False condition triggers ELSE action [OK]

Common Mistakes:

• Assuming Motor stays ON by default
• Confusing TRUE and FALSE conditions
• Thinking ELSE is optional and skipped

4. Identify the error in this PLC IF-THEN-ELSE code:

IF Temperature > 100 THEN
  Alarm := TRUE
ELSE
  Alarm := FALSE;
END_IF;

medium

A. Missing END_IF statement

B. Missing semicolon after Alarm := TRUE

C. Incorrect comparison operator

D. ELSE should be ELSEIF

Solution

1. Step 1: Check statement endings

   In PLC structured text, each assignment must end with a semicolon. 'Alarm := TRUE' is missing a semicolon.

2. Step 2: Verify other parts

   Comparison operator '>' is correct, END_IF is present, and ELSE is correct for two branches.

3. Final Answer:

   Missing semicolon after Alarm := TRUE -> Option B

4. Quick Check:

   Statements need semicolons [OK]

Hint: Every assignment ends with a semicolon [OK]

Common Mistakes:

• Omitting semicolons after assignments
• Confusing ELSE with ELSEIF
• Forgetting END_IF

5. You want a PLC program to turn ON a fan only if the temperature is above 30°C and the humidity is below 70%. Which IF-THEN-ELSE statement correctly implements this?

hard

A. IF Temperature > 30 AND Humidity < 70 THEN
Fan := ON;
ELSE
Fan := OFF;
END_IF;

B. IF Temperature > 30 OR Humidity < 70 THEN
Fan := ON;
ELSE
Fan := OFF;
END_IF;

C. IF Temperature > 30 THEN
Fan := ON;
ELSE IF Humidity < 70 THEN
Fan := ON;
ELSE
Fan := OFF;
END_IF;

D. IF Temperature > 30 THEN
Fan := ON;
END_IF;
IF Humidity < 70 THEN
Fan := ON;
END_IF;

Solution

1. Step 1: Understand the condition requirements

   The fan should turn ON only if both conditions are true: temperature above 30 and humidity below 70.

2. Step 2: Check each option's logic

   IF Temperature > 30 AND Humidity < 70 THEN Fan := ON; ELSE Fan := OFF; END_IF; uses AND, requiring both conditions. IF Temperature > 30 OR Humidity < 70 THEN Fan := ON; ELSE Fan := OFF; END_IF; uses OR, which turns fan ON if either condition is true. IF Temperature > 30 THEN Fan := ON; ELSE IF Humidity < 70 THEN Fan := ON; ELSE Fan := OFF; END_IF; turns fan ON if either condition is true via ELSE IF. IF Temperature > 30 THEN Fan := ON; END_IF; IF Humidity < 70 THEN Fan := ON; END_IF; turns fan ON separately for each condition, possibly turning ON incorrectly.

3. Final Answer:

   IF Temperature > 30 AND Humidity < 70 THEN Fan := ON; ELSE Fan := OFF; END_IF; -> Option A

4. Quick Check:

   Both conditions must be true -> use AND [OK]

Hint: Use AND to require both conditions true [OK]

Common Mistakes:

• Using OR instead of AND
• Using separate IFs causing wrong ON state
• Misusing ELSE IF for combined conditions